The present invention relates to a carrier frequency assignment system in a frequency division multiple access (FDMA) communication system.
A SPADE (Single channel per carrier PCM multiple Access Demand assignment Equiqment) system has been used in the INTELSAT for international communication. In that system, each PCM signal is modulated through PSK (Phase Shift Keying), and the modulated signal is assigned to a signal carrier frequency which is prepared on a frequency axis, pased on a demand or call basis.
FIG. 2 shows the frequency allocation in SPADE system. The frequencies are not pre-assigned to each earth station, but are shared commonly with all the earth stations which are included in the communication system, and when a satellite channel is requested, a carrier frequency (CH-3 through CH-399, or CH-3' through CH-399') is assigned. Each earth station can assign channels independently by the distributed control method. The SPADE system is advantageous when the number of earth stations is large, and the number of channels for each earth station is small.
Further, an FDMA system is expected to be adopted as a subscriber part in a future regenerative satellite communication system, in which an exchanger and a modem are installed on board the satellite.
In the systems described above, a single frequency band is assigned to each service with an unique transmission bit rate.
However, a plurality of transmission bit rate services will be required in the future because of the demand of a variety of services. A multiple bit rate of 64 kbit/s and/or sub-multiple bit rate of said 64 kbit/s which has been used in PCM transmission system might be required. Further, because of the adoption of various error correction techniques, a transmission bit rate would not solely depend upon service bit rate, but it would also depend upon error correction system.
In the INTELSAT business satellite communication system which accommodates a plurality of services, a frequency band continuous on the frequency scale is assigned to each service. FIG. 3 shows the example of frequency assignment in which the error correction with the coding rate of 3/4 and the 4 phase PSK modulation are applied. In the figure, the service with the information bit rate of 64 kbit/s is transferred to a PSK signal with the transmission bit rate of 94 kbit/s after the error correction with the coding rate of 3/4 and some necessary synchronization bits are attached. Similarly, the information bit rate 192 kbit/s is changed to the transmission bit rate 282 kbit/s, and 128 kbit/s changed to 188 kbit/s. The frequency bands for those transmission bit rates are 67.5 kHz, 202.5 kHz, and 135 kHz, respectively. Namely, a narrow frequency band is assigned to a low bit rate channel, and a wide frequency band is assigned to a high bit rate channel.
The continuous frequency assignment as shown in FIG. 3 has no problem only when a number of required channels is small and there is a surplus transmission band. However, as shown in FIG. 4, when the frequencies #1 through #5 have been assigned, and the #N high bit rate channel is required to be assigned, that #N frequency cannot be assigned, although the total of unused frequency bands is larger than the band which is necessary for assigning the #N channel. The reason is that the unused frequency bands are not continuous on the frequency scale.
That kind of problem, in which a channel can not be assigned even when the total of unused frequency bands is larger than the necessary band width, because the unused frequency bands are not continuous, will increase in future in proportion to the increase of traffic volume. In that situation, the system efficiency would be decreased considerably.